The present invention relates to a method as defined in the preamble to claim 1, a use of the method, an apparatus for executing the method and a computer-program product.
In numerous technical and medical procedures, it is critical that the exact position of a certain object be known. Whereas in medicine, the position of individual tissue elementsxe2x80x94such as a tumor, which is to be destroyed or limited in growth through irradiation-must be ascertained, the determination of the position for entry into a computer system, for example for xe2x80x9ccyberspacexe2x80x9d applications, is of less significance. In these applications, a position-detection or position-entry unit is also referred to as a three-dimensional mouse.
The international patent application by the same applicant, WO 97/36192, describes a known apparatus and a known method, respectively, for determining position. According to the known teaching, it is provided to create an alternating field with the aid of a field-generator unit. Depending on the number of degrees of freedom of a sensor element whose position is to be determined, a plurality of alternating fields can be superimposed over one another. A processing and control unit, which controls the field-generator unit, and processes the signals received from the sensor element, ascertains the position and possibly the location of the sensor unit. In this regard, the content of the above-cited publication constitutes an integral component of this description.
It has been shown that, in magnetic field-based positioning, as is used in, for example, the teaching known from WO 97/36192, eddy currents are generated in adjacent, electrically conductive objects. These currents lead to distortions in the original magnetic alternating field, and therefore to systematic errors. This means that, when the position and orientation of sensor elements in the distorted alternating field are ascertained as if no electrically conductive object were present, the obtained values are systematically skewed.
A method for compensating interfering effects caused by conductive objects is known by the name xe2x80x9cdistortion mapping.xe2x80x9d This method is described, for example, in an essay titled xe2x80x9cCalibration of Tracking Systems in a Surgical Environmentxe2x80x9d (Birkfellner et al., IEEE Trans Med Imaging, Vol. 17(5), pp. 737-742, 1998). In the known method, the position and orientation of a sensor element are likewise ascertained with the aid of a position-measurement system that employs magnetic field-based positioning. A second position-measurement system, which is insensitive to electrically conductive objects, is provided for compensating interfering effects. The difference between the positions and orientations determined with the two position-measurement systems is then used to correct the position and orientation determined with the aid of the magnetic field-based position-measurement system.
A drawback of the known method, however, is that, to attain high precision, the position and orientation difference must be measured at as many points as possible. To obtain additional points, a costly interpolation method must be used. The very high outlay is illustrated, in particular, by the following example: If a volume of 1 m3is supposed to be measured, specifically over 10 cm in all three axes and at ten different orientation angles, 10,000 points are obtained. Furthermore, the aforementioned second position-measurement system is necessary.
In another known method for compensating interfering effects, pulsed DC fields generate magnetic fields. Eddy-current effects are compensated by taking magnetic-field measurements after the eddy-current components contained in the measurement signal have decayed. More detailed explanations of the known method can be found in the publications U.S. 5,453,686 and U.S. 5,767,669. It has been seen that the precision of the obtained results is unsatisfactory. In particular, the compensation is incomplete if the decay times of the eddy-current components exceed the pulse time between two DC pulses. While this can be remedied by lengthening the pulse time, such a solution results in an undesired, lower measurement rate. Moreover, the known compensation method cannot be implemented in position-measurement systems that employ magnetic positioning and generate magnetic alternating fields.
It is therefore the object of the present invention to disclose a method that permits an improved determination of the position and/or the location of a sensor element.
This object is accomplished by the actions disclosed in the characterizing portion of claim 1. The additional claims disclose advantageous embodiments of the invention, an application of the method, an apparatus for executing the method and a computer-program product.
With the method according to the invention, it is possible to eliminate the influence of conductive objects, or at least reduce it significantly. This method is also more general and more precise than the known methods. Finally, the geometry-dependent portion of the calculations can be executed in the sense of a system calibration prior to the actual implementation of the position-measurement system.